I'm faced with a similar choice, and I'm still exploring a number of ideas:
although I also don't want to have in-app purchases in my Base app (which currently needs no permissions at all), I'm thinking that having in-app purchases in the Normal app will be acceptable, so the user installs the Base app (which holds all the Base + Normal code), and the first upgrade is to install the Normal app, then future upgrades are achieved by running the very limited upgrade code from the Normal app.
I intend that the user does not see multiple apps on their device - the Normal app will not declare any launcher intent, so will be hidden from the user
I'm looking to have the Base app change it's declared launcher activity so that the icon changes from Base to Normal (or beyond)
By keeping in-app billing out of the Base app, I keep the ability of that app to work on Kindle Fire and other devices outside the Google Play ecosystem.